The endomembrane system, consisting of nuclear envelope, endoplasmic reticulum, Golgi aparatus, secretory vesicles and plasma membrane, functions in both the synthesis and exocytosis of secretory products from eukaryotic cells. While the functions of endomembrane components in formation, compartmentalization and exocytosis of products has been extensively investigated, it is widely acknowledged that much remains to be learned about the integrated function of the endomembrane system. In the highly specialized mammary epithelial cell, exocytosis of milk constituents involves the net discharge of cell surface membrane. Since the epithelial cell must accomplish a considerable net synthesis of new plasma membrane, the lactating mammary gland affords a unique opportunity to study endomembrane formation, differentiation and flow. The study proposed herein is designed (1) to characterize the membrane and contents of Golgi apparatus-derived secretory vesicles, (2) to determine if the vesicle membrane is identical to and capable of replenishing the apical cell surface membrane, (3) to compare the rates of appearance and turnover of individual membrane and secretory constituents in the various endomembrane system components, and (4) to determine the extent of and enzymatic basis for modification of secretory and membrane proteins during their passage through the endomembrane system. Approaches used will emphasize biochemical studies of purified and cytologically characterized subcellular fractions. Results obtained will further understanding of the mechanisms involved in secretion of the constituents of milk and will yield information on the role of intracellular endomembranes in renewal of the cell surface.